[IMP] mail: slightly improve tool to call mail gateway in tests
[IMP] mail: slightly improve tool to call mail gateway in tests

We now also support formatting a return path when emulating incoming emails.
Purpose is to integrate some more tests in a near future about incoming
emails, notably in CRM.

This commit is a preliminary cleaning coming from task 2088565 about CRM
onchange to compute methods update.

LINKS

Task ID 2088565
Community PR #42344
Enterprise PR odoo/enterprise#7403
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